Today I started upgrading my hosts to 5.1 from 5.0. I have yet to find a quick guide on upgrading. First, I can’t drive out to my datacenter and spend a day there to upgrade the hosts, so I need to do it remotely. Also I don’t really know the esxi terminal really well, so this solution will use the update manager plugin.  I have a smaller vmware environment, about 15 hosts so this worked for me.
First download VMware-VIMSetup-all-5.1.0-799735.iso. The 3.4GB iso. Move it to your machine or vm that runs vCenter Server and mount it.

Run the simple installer. It will throw a bunch of warnings, it scared me, but I accepted the defaults and clicked through. The installer will prompt for a new password. I’m not entirely sure what this is used for.  The installer will take about 20-40 minutes depending on if its installing SQL. Then you want to install the update manager

Now you are finally ready to import the 5.1 image.
If you haven’t downloaded it – VMware-VMvisor-Installer-5.1.0-799733.x86_64.iso.

You want to upload the iso to the update manager like this…

Then you can attach and remediate as normal, your host will be upgraded to 5.1